An actuator is a functional device for moving or controlling a mechanism or system. The actuator can be classified as an active or semi-active actuator. The active actuator, such as the electric motor, consumes a large amount of power and may have difficulties to stop stopping a torque output thereof immediately in the event of excessive output occurrence. The most widely used methods for motor braking are either injecting inverse direct current or using external brakes or choppers. These methods cause high power consumption and could damage the devices in prolonged usage.
MR fluids-based actuators work well in semi-active means while consuming little power. MR fluids are suspensions consisting of iron particles or powders in a low-permeability base fluid. Under the influence of a magnetic field, a rheological behavior of the fluids is changed. The flow resistance increases considerably with a growing magnetic flux density. The rheological properties, such as the viscosity and shear stress of these fluids, change rapidly within a few milliseconds, and are reversible when the magnetic field is applied.
MR fluids possess some advantages. They may produce a high yield stress subject to a magnetic field while consuming little power. They also have good stability and a fast response time. In addition, it they may work in a broad range of temperatures. The applications of MR fluids have been considered in a variety of energy dissipation and transfer devices, such as shock absorbers, clutches, brakes, engine mounts, and so on. In the above applications, an electromagnetic element is utilized as the source of the magnetic field. Through changing the current applied to the coil, the magnetic field can be changed accordingly and so does the yield stress produced by MR fluids.
Meanwhile, with the development of science and technology, many sophisticated engineering applications, such as robots and medical devices, need actuation systems to perform desired complicated tasks. Actuators used in these applications are desired to have more than one function in many cases. However, no actuator functions as a motor, a generator, a clutch and a brake in one device.